An amoeba,Hartmannella castellanii,which possesses many features typical of higher‐order animal cells, was irradiated with 1‐MHz ultrasound while suspended in ordinary growth medium and in one with increased viscosity. The ultrasonically produced cavitation was monitored and a strong correlation is found between the number of discrete cavitation events occurring and the decrease in cell numbers, on irradiating at 515 W/cm2for 10 min. The growth of treated cells was also examined.